The invention relates to a narrow-band amplifier the input line of which is provided on a printed circuit board, at least partly, very close to a line at the output of the amplifier.
With amplifier circuits provided on a printed circuit board, it is often unavoidable, inter alia because of the available space, that input and output lines of the amplifier are provided extremely closely to each other on the printed circuit board. This may cause a portion of the signal energy to be fed back from the output to the input, which may result in self-excitation when the input signal and the fed-back output signal have the same phase. Effects of such a type may be particularly pronounced when a parallel-resonant circuit is provided at the input of the amplifier and the input line is coupled to an output line through which the current through one of the reactances flows and/or when a parallel-resonant circuit tuned to the same frequency is connected to the input circuit.